The invention relates to an NMR (=nuclear magnetic resonance) DNP (=dynamic nuclear polarization)-MAS (=magic angle spinning) probe head comprising an MAS stator for receiving an MAS rotor having a sample substance in a sample volume, and a hollow microwave waveguide for feeding microwave radiation through an opening of the microwave waveguide into the sample volume, an axially expanded rod-shaped microwave coupler made of dielectric material and located in the opening of the microwave waveguide. In this context, “rod-shaped” means that the length in the rod axis is longer than the extent in the directions orthogonal thereto.
Such a configuration is known from DE 10 2008 009 376 A1 (=reference [0]). Reference [5] shows a similar configuration, but without an axially expanded rod-shaped microwave coupler made of dielectric material and located in the opening of the microwave waveguide.
Magnetic Nuclear Resonance with Magic Angle Spinning (=MAS-NMR)
In a magic angle spinning (MAS) device for performing experiments based on magnetic nuclear resonance (NMR), a sample to be examined is placed in a cylindrical MAS rotor inside an MAS stator and pneumatically made to rotate. NMR excitation and detection are performed in the normal way by means of a solenoid, into the center of which the rotor is inserted cylinder-symmetrically.
Dynamic Nuclear Polarization (=DNP)
In the field of nuclear magnetic resonance spectroscopy, there are experimental methods that enable a substantial increase in nuclear polarization and therefore in the detection sensitivity of the experiment. One of these methods is dynamic nuclear polarization (=DNP). This method requires simultaneous irradiation of a magnetic microwave field for polarization of electron spins at a frequency that is higher by a factor of 660 than the Larmor frequency of the 1 H nuclear spins.
A typical DNP arrangement consists of an NMR coil that is tuned to a nuclear Larmor frequency (e.g. 1H—400 MHz) and simultaneous irradiation of a microwave field at 263 GHz. DNP configurations are described in reference [7], for example.
MAS-DNP-NMR
When MAS-NMR is combined with dynamic nuclear polarization (DNP) to increase the signal, the sample must additionally be irradiated with an electromagnetic wave in the upper microwave/sub-THz range. This approach is described in references [2] to [6].
The microwave/sub-THz beam is guided along a microwave waveguide to the region of the sample and radiated onto the rotor and the sample through an open end either perpendicularly to the axis of the rotor (radial)—see references [3] to [5]—or parallel thereto (axial)—see references [2] and [6]. The spatial extent of the beam perpendicular to the direction of propagation is normally greater than the surface of the rotor, so that only part of the radiated energy is used for the DNP effect. In the case of radial irradiation, therefore, a change in the beam shape was achieved with a lens made of dielectric material that was better adapted to the cross-sectional area of the rotor—see references [1] and [3].
Because of the distance of the radiating waveguide aperture from the sample and the divergence of the microwave/sub-THz beam, the cross section of the beam at the sample location is substantially greater in at least one dimension than the cross section of the sample, so that the energy density at the sample location is reduced. There are also scattering effects of the beam at dielectric interfaces that can reduce the field strength further. In order to nevertheless produce the necessary field strength in the sample, a complex high-power microwave source has to be used—see references [4] to [6]. In the microwave coupler according to the reference cited above [0] (=DE 10 2008 009 376 A1), TE modes are to be coupled into the resonator. For this purpose, the microwave coupler is flush with the walls, which is physically necessary for this application. However, this is neither necessary nor helpful for coupling in the HE 11 mode.
The object of this invention is to provide an NMR DNP-MAS probe head of the type defined in the introduction, with which focused and therefore more efficient microwave irradiation into the sample volume is possible and in which the sample is polarized in the most homogeneous possible way, wherein, in particular, the HE 11 mode is to be optimally coupled.